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|Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible Disk Nature of Bright Early-Type Dwarfs|
We present a systematic search for disk features in 476 Virgo Clusterearly-type dwarf (dE) galaxies. This is the first such study of analmost-complete, statistically significant dE sample, which includes allcertain or possible cluster members with mB<=18 that arecovered by the optical imaging data of the Sloan Digital Sky Survey DataRelease 4. Disk features (spiral arms, edge-on disks, or bars) wereidentified by applying unsharp masks to a combined image from threebands (g, r, and i), as well as by subtracting the axisymmetric lightdistribution of each galaxy from that image. Fourteen objects areunambiguous identifications of disks, 10 objects show ``probable disk''features, and 17 objects show ``possible disk'' features. The numberfraction of these galaxies, for which we introduce the term ``dEdi,''reaches more than 50% at the bright end of the dE population anddecreases to less than 5% for magnitudes mB>16. Althoughpart of this observed decline might be due to the lower signal-to-noiseratio at fainter magnitudes, we show that it cannot be caused solely bythe limitations of our detection method. The luminosity function of ourfull dE sample can be explained by a superposition of dEdis and ordinarydEs, strongly suggesting that dEdis are a distinct type of galaxy. Thisis supported by the projected spatial distribution: dEdis show basicallyno clustering and roughly follow the spatial distribution of spirals andirregulars, whereas ordinary dEs are distributed similarly to thestrongly clustered E/S0 galaxies. While the flattening distribution ofordinary dEs is typical for spheroidal objects, the distribution ofdEdis is significantly different and agrees with their being flat oblateobjects. We therefore conclude that the dEdis are not spheroidalgalaxies that just have an embedded disk component but are instead apopulation of genuine disk galaxies. Several dEdis display well-definedspiral arms with grand-design features that clearly differ from theflocculent, open arms typical for late-type spirals that have frequentlybeen proposed as progenitors of dEs. This raises the question of whatprocess is able to create such spiral arms-with pitch angles like thoseof Sab/Sb galaxies-in bulgeless dwarf galaxies.
|The origin of H I-deficiency in galaxies on the outskirts of the Virgo cluster. II. Companions and uncertainties in distances and deficiencies|
The origin of the deficiency in neutral hydrogen of 13 spiral galaxieslying in the outskirts of the Virgo cluster is reassessed. If thesegalaxies have passed through the core of the cluster, their interstellargas should have been lost through ram pressure stripping by the hotX-ray emitting gas of the cluster. We analyze the positions of these HI-deficient and other spiral galaxies in velocity-distance plots, inwhich we include our compilation of velocity-distance data on 61elliptical galaxies, and compare with simulated velocity-distancediagrams obtained from cosmological N-body simulations. We find that20% relative Tully-Fisher distance errors are consistent with thegreat majority of the spirals, except for a small number of objectswhose positions in the velocity-distance diagram suggest grosslyincorrect distances, implying that the Tully-Fisher error distributionfunction has non-Gaussian wings. Moreover, we find that the distanceerrors may lead to an incorrect fitting of the Tolman-Bondi solutionthat can generate significant errors in the distance and especially themass estimates of the cluster. We suggest 4 possibilities for theoutlying H I-deficient spirals (in decreasing frequency): 1) they havelarge relative distance errors and are in fact close enough (atdistances between 12.7 and 20.9 Mpc from us) to the cluster to havepassed through its core and seen their gas removed by ram pressurestripping; 2) their gas is converted to stars by tidal interactions withother galaxies; 3) their gas is heated during recent mergers withsmaller galaxies; and 4) they are not truly H I-deficient (e.g. S0/amisclassified as Sa).Appendix A is only available in electronic form athttp://www.edpsciences.org
|The faint end of the galaxy luminosity function|
We present and discuss optical measurements of the faint end of thegalaxy luminosity function down to MR=-10 in five differentlocal environments of varying galaxy density and morphological content.The environments we studied, in order of decreasing galaxy density, arethe Virgo Cluster, the NGC 1407 Group, the Coma I Group, the Leo Groupand the NGC 1023 Group. Our results come from a deep wide-angle surveywith the National Astronomical Observatories of Japan Subaru 8-mTelescope on Mauna Kea and are sensitive down to very faintsurface-brightness levels. Galaxies were identified as group or clustermembers on the basis of their surface brightness and morphology. Thefaintest galaxies in our sample have R~ 22.5. There were thousands offainter galaxies but we cannot distinguish cluster members frombackground galaxies at these faint limits so do not attempt to determinea luminosity function fainter than MR=-1010. In all cases,there are far fewer dwarfs than the numbers of low-mass haloesanticipated by cold dark matter theory. The mean logarithmic slope ofthe luminosity function between MR=-1018 andMR=-1010 is α~=-1.2, far shallower than the cold darkmatter mass function slope of α~=-1.8. We would therefore need tobe missing about 90 per cent of the dwarfs at the faint end of oursample in all the environments we study to achieve consistency with CDMtheory. It is unlikely that such large numbers of dwarfs are missedbecause (i) the data are deep enough that we are sensitive to very lowsurface brightness galaxies, and (ii) the seeing is good enough that wecan have some confidence in our ability to distinguish high surfacebrightness dwarfs from background galaxies brighter than R= 22.5. Onecaveat is that we miss compact members taken to be background galaxies,but such objects (like M32) are thought to be rare.
|The luminosity function of the Virgo Cluster from MB=-22 to -11|
We measure the galaxy luminosity function (LF) for the Virgo Clusterbetween blue magnitudes MB=-22 and -11 from wide-fieldcharge-coupled device (CCD) imaging data. The LF is only graduallyrising for -22
|New clues to the evolution of dwarf early-type galaxies|
Surface photometry of 18 Virgo cluster dwarf elliptical (dE) and dwarflenticular (dS0) galaxies, made by Gavazzi et al. in the H band(1.65μm) and in the B band (0.44μm), shows that the ratio of theeffective radii of these stellar systems in the B and H bands, r eB /r eH , ranges between 0.7 and 2.2. In particular,dwarf ellipticals and lenticulars with a red total colour index B -H(i.e. with 3.2
|Spectrophotometry of Galaxies in the Virgo Cluster. I. The Star Formation History|
As a result of an extensive observational campaign targeting the VirgoCluster, we obtained integrated (drift-scan mode) optical spectra andmultiwavelength (UV, U, B, V, H) photometry for 124 and 330 galaxies,respectively, spanning the whole Hubble sequence, and withmp<=16(Mp<=-15). These data were combined toobtain galaxy Spectral Energy Distributions (SEDs) extending from 2000to 22000 Å. By fitting these SEDs with synthetic ones derivedusing Bruzual & Charlot population synthesis models we try toconstrain observationally the star formation history (SFH) of galaxiesin the rich cluster of galaxies nearest to us. Assuming a Salpeter IMFand an analytical form for the SFH, the fit free parameters are the age(T) of the star formation event, its characteristic timescale (τ),and the initial metallicity (Z). In this work we test the (simplistic)case in which all galaxies have a common age T=13 Gyr, exploring a SFHwith ``delayed'' exponential form (which we call ``a la Sandage''), thusallowing for an increasing SFR with time. This SFH is consistent withthe full range of observed SEDs, provided that the characteristictimescale τ is let free to vary between 0.1 (quasi-instantaneousburst) and 25 Gyr (increasing SFR) and Z between 1/50 and 2.5 Zsolar.Elliptical galaxies (including dEs) are best fitted with shorttimescales (τ~3 Gyr) and metallicity varying between 1/5 and Zsolar.The model metallicity is found to increase as a function of H-bandluminosity. Spiral galaxies require that both τ and metallicitycorrelate with H-band luminosity: low-mass Im+BCD have subsolar Z andτ>=10 Gyr, whereas giant spirals have solar metallicities andτ~3 Gyr, consistent with elliptical galaxies. Moreover, we find thatthe SFH of spiral galaxies in the Virgo Cluster depends upon thepresence at their interior of fresh gas capable of sustaining the starformation. In fact, the residuals of the τ vs. LHrelation depend significantly on the H I content. H I deficient galaxieshave shorter (up to a factor of 4) τ (truncated SFH) than spiralswith normal H I content. Based on observations collected at theObservatoire de Haute Provence (OHP) (France), operated by the CNRS,France, and at the European Southern Observatory (Chile) (programme66.B-0026).
|Luminosity Functions of 10 Nearby Clusters of Galaxies. I. Data|
Wide field CCD imaging was carried out in the RC band for 10nearby clusters of galaxies and four control fields to derive the totaland type-specific luminosity functions. Observation and data reductionprocedures are described. We extract galaxies down to RC~20mag and classify them into two broad types, r1/4-like andexponential-like, on the basis of the bulge-to-total luminosity ratioB/T estimated from Petrosian quantities. We describe our classificationscheme in detail. We apply a single classification scheme to both giantand dwarf galaxies. The consistency of our classification is verifiedfor giant galaxies using both simulated images and real data in theliterature. We set the boundary of our two types at B/T=0.35. Thisboundary gives 70% completeness to both the r1/4-like sample(for E/S0 galaxies) and the exponential-like sample (for Sa-Irrgalaxies). Our classification for dwarf galaxies is investigated usinghigher resolution images of some 20 dwarf galaxies in the Virgo Cluster.Galaxy catalogs are constructed, which include position, magnitude, andB/T. The projected sky distribution of each type of galaxies is shownfor the clusters and control fields.
|Galaxy Populations and Evolution in Clusters. I. Dynamics and the Origin of Low-Mass Galaxies in the Virgo Cluster|
Early-type dwarfs are the most common galaxy in the local universe, yettheir origin and evolution remain a mystery. Various cosmologicalscenarios predict that dwarf-like galaxies in dense areas are the firstto form and hence should be the oldest stellar systems in clusters. Byusing radial velocities of early-type dwarfs in the Virgo cluster wedemonstrate that these galaxies are not an old cluster population buthave signatures of production from the infall of field galaxies.Evidence of this includes the combined large dispersions andsubstructure in spatial and kinematic distributions for Virgo early-typedwarfs and a velocity dispersion ratio with giant ellipticals expectedfor virialized and accreted populations. We also argue that thesegalaxies cannot originate from accreted field dwarfs, but must havephysically evolved from a precursor population, of different morphology,that fell into Virgo some time in the past.
|1.65 μm (H-band) surface photometry of galaxies. VII. Dwarf galaxies in the Virgo Cluster|
We present near-infrared H-band (1.65 μm) observations and surfacebrightness profile decompositions for 75 faint (13.5 <~ mp<~ 18.5) galaxies, primarily taken among dwarf Ellipticals members ofthe Virgo cluster, with some Centaurus Cluster members, a BCD and twopeculiar galaxies taken as fillers. We model their surface brightnessprofiles with a de Vaucouleurs (D), exponential (E), mixed (bulge+diskor M) or truncated (T) law, and we derive for each galaxy the H bandeffective surface brightness (μe) and effective radius(re), the asymptotic total magnitude HT and thelight concentration index C31, defined as the ratio betweenthe radii that enclose 75% and 25% of the total light HT. Fora subsample we compare the NIR surface photometry with similar datataken in the B and V bands, and we give the B-H and B-V color profiles.Combining the present data with those previously obtained by our group(1157 objects) we analyze the NIR properties of a nearly completesample, representative of galaxies of all morphological types, spanning4 decades in luminosity. We confirm our earlier claim that the presenceof cusps and extended haloes in the light profiles (C31>5)is a strong, non-linear function of the total luminosity. We also findthat: i) among dE and dS0 galaxies D profiles are absent; 50% of thedecompositions are of type M, the remaining being of type E or T. ii)Less than 50% of the giant elliptical galaxies have pure D profiles, themajority being represented by M profiles. iii) Most giant galaxies (fromelliptical to Sb) have M profiles. iv) Most of late type spirals (Scd toBCD) have either E or T profiles. v) The type of decomposition is astrong function of the total H band luminosity, independent of theHubble classification: the fraction of type E decompositions decreaseswith increasing luminosity, while those of type M increase withluminosity. Pure D profiles are absent in the low luminosity rangeLH<1010 Lsolar and become dominantabove 1011 Lsolar, while T profiles are presentonly among low luminosity galaxies. vi) We find that dE-peculiargalaxies have structural parameters indistinguishable from those oflate-type dwarfs, thus they might represent the missing link between dEsand dIs. Based on observations taken with the ESO/NTT (ESO program64.N-0288), with the Telescopio Nazionale Galileo (TNG) operated on theisland of La Palma by the Centro Galileo Galilei of the CNAA at theSpanish Observatorio del Roque de los Muchachos of the IAC, with the SanPedro Martir 2.1~m telescope of the Observatorio Astronomico Nacional(OAN, Mexico), and with the OHP 1.2~m telescope, operated by the FrenchCNRS.
|Blue compact dwarf galaxies and new velocities in Virgo|
We present new spectral observations of 303 galaxies brighter thanB_J=17.6 in the central 30 deg^2 of the Virgo cluster field. Thegalaxies were selected from two overlapping samples designed for asearch for blue compact dwarf (BCD) galaxies and for a more generalstudy of Virgo dwarf elliptical (dE) galaxies. Many of these galaxieswere designated `background' (i.e., not members of the Virgo cluster) onmorphological grounds by Binggeli, Sandage & Tammann in their VirgoCluster Catalog (VCC). Our principal aim was to measure radialvelocities of these galaxies in order to determine which were members ofthe cluster, looking in particular for any cluster members previouslymisclassified as background galaxies. We measured reliable velocitiesfor 291 galaxies, of which nine were found to be cluster members. Fiveof these were listed in the VCC as `members' and four as `possiblemembers'. We also confirmed that 10 VCC `background' galaxies werecorrectly identified, and determined that an additional three VCC`possible members' were background galaxies. These results show that theVCC membership estimates were generally correct. Our sample of candidateBCD galaxies was defined by objective criteria from digitized platematerial and therefore allows us to place new limits on the dwarf-galaxypopulation. We confirm the drop in the BCD luminosity function at anabsolute magnitude of M_B=-14 reported by Binggeli, Sandage &Tammann (using their Virgo distance modulus). We also show that if BCDsfade at the end of the current burst of star formation, such `dead'BCDs, if they exist, would all have to be fainter than our sample limitof B_J=17.6. We use a (U-B_J)(B_J-R_F) colour-colour diagram plottedfrom our photographic data to separate the different dwarf-galaxy typeseffectively and use this colour information to argue that there are noblue star-forming progenitors of dE galaxies in the cluster. Most of thegalaxies of compact appearance we selected were background objects whichwe show to be significantly younger than general field galaxies; thissample also shows considerable clustering in redshift space.
|An image database. II. Catalogue between δ=-30deg and δ=70deg.|
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|Distances to 64 Virgo dwarf-elliptical galaxies and the depth in their spatial distribution|
We derive distances for 64 dwarf ellipticals (dEs) in the direction ofthe Virgo cluster's (VC) core, by means of the luminosity-profilecurvature (L-n) relationship and by means of their global scalelengths,which we find to be correlated with the shapes of theirsurface-brightness profiles. The great depth we find in the spatialdistribution of Virgo dEs is not consistent with a unimodal distributiondue to a single spherically symmetric concentration of galaxies. Thisdepth is also sufficient to explain much of the disagreement over theVC's distance, and thereby much of the Hubble-constant (H_0)controversy.
|Near Infrared Imaging of Dwarf Ellipticals Irregulars and Blue Compact Galaxies in the Virgo Cluster|
New near-IR images are presented for 13 dwarf galaxies in the Virgocluster. Together with previous data these provide a data base of JHKimaging for 26 dwarf ellipticals (dEs), dwarf irregulars (dIs) and bluecompact dwarfs (BCDs). These images show the dIs to be highly asymmetricand unrelaxed, implying that they are dynamically young and unevolved.This is consistent with their blue near-IR and optical-IR colours whichare most easily explained by young stellar populations. The dEs aresymmetrical and apparently relaxed, with very uniform colours indicatingthat they are dominated by old stars. They generally have exponentiallight profiles, but the brighter galaxies tend to exhibit more cuspedlight distributions, similar to the de Vaucouleurs profiles of brightellipticals. The BCDs have moderately asymmetric light profiles, andparadoxically red colours, possibly indicating an intermediate-agestellar population. They are probably dEs which have undergone bursts ofstar formation in the last few X 10^9^ years, whilst the dIs are afundamentally distinct population. Colour gradients are present in manyof the galaxies, invariably in the sense that the nuclei are redder thanthe surrounding galaxy light.
|The surface brightness test for the expansion of the universe. II - Radii, surface brightness, and absolute magnitude correlations for nearby E galaxies|
Data for elliptical galaxies in the Virgo, Fornax, and Coma clusters andin the general field are analyzed in order to determine the dispersionin average surface brightness. The data are discussed using measures ofboth the effective radius and the Petrosian r(eta) radii. The dispersionis found to be about 0.5 mag after reducing the data to absolutemagnitude M(B) = -22. This value is smaller than the 1.8 mag Tolman (1 +z) exp 4 factor, even at the modest redshift of z = 0.5, showing thatthe Tolman test is feasible in practice as well as in principle.
|Surface photometry of dwarf elliptical galaxies in the Virgo cluster|
Photographic surface photometry is carried out for 69 dwarf ellipticalsin the central region of the Virgo Cluster, using two plates in the Bband taken with the 2.5 m du Pont telescope at Las Campanas Observatory.Luminosity profiles and various photometric parameters are obtained, andthe following three results are derived: (1) All the dwarf ellipticalsin the sample have luminosity profiles consistent with exponential lawsexcept for six galaxies which are probably giant ellipticals in thebackground. If they are physcial members of the Virgo Cluster, they maybe identified with 'classical' dwarf ellipticals recognized by Wirth andGallagher. (2) The distribution of apparent flattenings of dwarfellipticals suggests that the population of dwarf ellipticals is notdominated by flat disk systems like spiral galaxies, in spite of theirexponential profiles. (3) Two important diagrams, a diagram ofconcentration index versus absolute magnitude and a diameter versussurface brightness diagram (DSBD), show a hint of structuraldiscontinuity in the sequence of spheroidal stellar systems consistingof giant ellilpticals, dwarf ellipticals, and globular clusters.
|Studies of the Virgo Cluster. II - A catalog of 2096 galaxies in the Virgo Cluster area.|
The present catalog of 2096 galaxies within an area of about 140 sq degapproximately centered on the Virgo cluster should be an essentiallycomplete listing of all certain and possible cluster members,independent of morphological type. Cluster membership is essentiallydecided by galaxy morphology; for giants and the rare class of highsurface brightness dwarfs, membership rests on velocity data. While 1277of the catalog entries are considered members of the Virgo cluster, 574are possible members and 245 appear to be background Zwicky galaxies.Major-to-minor axis ratios are given for all galaxies brighter than B(T)= 18, as well as for many fainter ones.
|A catalog of dwarf galaxies in Virgo|
A catalog listing the location, apparent angular diameter, type,estimated central light concentration, and estimated brightness of 846dwarf galaxies in a 200-deg-sq region in Virgo is presented. Thegalaxies comprise 634 ellipticals, 137 IC-3475-type galaxies, 73 dwarfspirals and irregulars, and two objects which are jets of normalgalaxies, and were found on nine long-exposure IIIa-J-emulsion platesmade with the 1.2-m-Schmidt telescope at Palomar Observatory from 1971to 1976. Concordances to other catalogs, tables of additionalparameters, maps, graphs, and photographs are provided. The projecteddistributions of normal and dwarf galaxies and the dependence ofapparent luminosity on central light concentration are discussed. It isfound that dwarf ellipticals and IC-3475-type galaxies are probablemembers of the Virgo cluster, while dwarf spirals and possibly dwarfirregulars are not.
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